1. Technical Field
The present invention relates to methods and apparatus for cancelling acoustic noise and, more particularly, to fluidic drivers for effecting noise cancellation.
2. Discussion of the Prior Art
Aircraft noise pollution is a topic of much debate and the subject of much research as well as legislation. The imposition of the Federal Aviation Administration (FAA) Stage 3 noise thresholds is a good example of this. Leading experts (Fotos, C. P., "Industry Experts Say NASA Must Devote More Resources to Civil Aeronautics," Aviation Week and Space Technology, p. 42, Feb. 24, 1992), however, agree that current quieting and control technology will be inadequate if stage three levels are to be met or exceeded economically. As by-pass ratios, and hence fan sizes, increase, turbofan engine fan noise components also increase. Passive noise reduction has been quite successful with significant reductions in fan tone levels, however, in the future, only incremental improvements can be expected to occur, because the much shorter inlet length state-of-the-art engines will not be able to accommodate the increased passive liners which would only have restricted space. As a result industry is looking to active control techniques to provide the necessary reduction in noise levels.
Active control of sound has shown great promise for a number of many applications (Williams, J. E. F., "Anti-Sound," Proc. Roy. Soc., A 395, pp. 63-88, 1984; Fuller, C. R., et al., "Active Structural Acoustic Control with Smart Structures," Proc. SPIE Conference on Fiber Optic Smart Structures and Skins II, pp. 338-358, 1989; and, Elliot, S. J., and Nelson, P. A., "The Active Control of Sound," Electronics and Communication Engineering Journal, pp. 127-136, August 1990). Examples of the use of active noise cancellation can be found in such day-to-day applications as audio systems including microphones and headphones that eliminate background noise. The basic principle behind active noise suppression is that of destructive interference. Unwanted sounds are cancelled out by out-of-phase interaction with a control sound generated by acoustic drivers operated by sophisticated computer algorithms that predict the required amplitude and phase. In particular, noise that has a well-defined periodic nature is readily attenuated. By measuring the amplitude and phase of the unwanted signal, and then generating counter-sound that is 180.degree. out of phase and projecting the counter-sound into the field, reductions of as much as an order of magnitude in sound pressure level can be achieved.
Research performed by the Virginia Polytechnic Institute (VPI), under NASA-Langley sponsorship, using conventional acoustic driver technology (i.e., very heavy compression drivers) is described in Thomas, R. J., Burdisso, R. A., Fuller, C. R., O'Brien, W. F., "Active Control of Fan Noise from a Turbofan Engine," AAIA No. 93-0597, 31st Aerospace Sciences Meeting & Exhibit, Jan. 11-14, 1993, pp. 1-9. The entire disclosure in this Thomas et al publication is incorporated herein by reference. The tests described therein have conclusively demonstrated that the periodic whine of turbofan noise (both primary frequency and first harmonic) from a real, commercial engine (Pratt and Whitney JT15D-1) radiated forward from the inlet, can be successfully reduced by as much as 20dB both on-axis as well as within a 60.degree. forward angle. However, in any practical application, the heavy and expensive compression type acoustic drivers, and awkward, long, radially disposed, exponential horns used in that preliminary research would not be sufficiently rugged and reliable to withstand the real environment. In future engines, with lower blade passage frequencies, even larger and heavier electronic drivers would have to be used, and the poor reliability of the moving parts would be a problem in commercial engines. In addition, the electrical power requirement to drive these compression drivers would require a dedicated source of electrical power.
Fluidic control systems in operational turbofan engine applications such as the thrust reverser control on the General Electric CF-6 engine, using compressor-bleed air for its power, have demonstrated incredible reliability as measured by a mean-time-before-unscheduled maintenance in excess of 650,000 hours. This performance, demonstrative of the reliability one might expect of aerospace applications of fluidics, is orders of magnitude better than that of conventional electro-mechanical systems.
Sound can be amplified fluidically, more specifically by acousto-fluidic amplifiers, as disclosed in co-pending U.S. patent application Ser. No. 08/340,899, filed Nov. 15, 1994, now U.S. Pat. No. 5,540,248, by Drzewiecki and Phillippi and entitled "Fluidic Sound Amplification System". The entire disclosure in that patent is expressly incorporated herein. In particular, low level sound waves provided by a low power electro-mechanical source, such as a headset earphone, impinge on a high velocity gas power jet and deflect it slightly, producing a larger deflection downstream. This results in larger recovered pressure changes than the pressure changes in the low level sound at the root of the jet, resulting in amplification as well-known in the art of fluidics. By serially amplifying the signal with several acousto-fluidic amplifier stages in series, where the output pressure of one stage drives the next, acoustic gain of the order of 1000:1 (60dB) or greater is readily attained. Because the dynamic response of these fluidic amplifiers depends to a great extent on the time it takes the power jet fluid to transit from the power jet nozzle exit to the output channels, which can be as low as several (10-100) microseconds, the frequency response of amplifiers staged in such a manner can be in excess of 10,000 Hz. By feeding the amplified output sound into a compact (folded or coiled) horn which matches the impedance of the acousto-fluidic amplifier output to the surrounding atmosphere, the sound can be transmitted to the outside or ambient environment with little loss in power or sound level. Since fluidic amplifiers are comparatively light in weight, inexpensive and have no moving parts, they are particularly attractive for these types of applications.